US20080148573A1 disclosed a rotary shaver including an outer cutter frame provided on a shaver main body. An outer cutter is installed in the outer cutter frame and has ring-shaped thin layer portions whose upper surfaces are shaving surfaces. The thin layer portions comprise hair introduction openings. The rotary shaver also comprises an inner cutter with cutter blades. The inner cutter is rotatable, wherein the cutter blades cooperate with the lower surface of the thin layer portions of the outer cutter to cut hair that entered the hair introduction openings. Each of these hair introduction openings is formed along a straight line which is slanted by a constant shearing angle in the rotational direction of the inner cutter.
The shaving surfaces, the lower surface of the thin layer portions as well as the upper edges of the cutter blades extend perpendicularly to the central axis.
The upper cutting edges of the cutter blades are directed towards the lower surface of the thin layer portions of the outer cutter. When cutting a hair, a cutting edge of the cutter blade contacts the hair at a first contact position whilst the cutting edge of a wall defining the hair introduction opening contacts the hair at a second contact position.
In practice there will be a relatively small cutting gap between the lower surface of the outer cutter and the cutting edge of the cutter blade, due to which the first contact position is located further away from the shaving surfaces than the second contact position. Due to the difference in distance to the shaving surface, the forces exerted during the cutting of the hair on the cutting edge of the cutter blade as well as on the cutting edge of the outer cutter, will introduce a torque forcing the cutting edges apart. When the distance between the cutting edges of the inner cutter and outer cutter becomes too large, the cutting of the hair is impeded. To keep the cutting gap as small as possible and theoretically even zero for a good cutting result, an axial force needs to be applied to press the cutting edges of the inner cutter against the lower surface of the outer cutter. However, by pressing the cutting edges of the inner cutter against the lower surface of the outer cutter, larger forces are needed to rotate the inner cutter with respect to the outer cutter. Furthermore, these forces may result in wear of the inner and outer cutter, as well as in heat generation causing less comfort during shaving.
U.S. Pat. No. 3,225,440 discloses an electric shaver having a dome-shaped external cutting member with a plurality of triangularly shaped hair-entry apertures, and having an internal cutting member with two straight radially extending cutting elements each comprising a curved cutting edge matching the dome-shaped external cutting member. Seen in the rotational direction of the internal cutting member, the triangular hair-entry apertures are successively arranged with their base portions facing towards and away from the rotational axis of the internal cutting member. As a result, the rotating cutting edges respectively enclose a positive and a negative shearing angle with the co-operating cutting edges of each two successive hair-entry apertures.
U.S. Pat. No. 3,889,372 discloses a blade assembly for an electric dry shaver including an outer blade and an inner blade rotatable to shave hairs in cooperation with the outer blade. The outer blade has a plurality of slits extending substantially in radial direction. The inner blade and the slits in the outer blades are so arranged that the intersection of their cutting edge lines define an optimum hair holding or cutting angle which is constant at all positions over the blade surface.